Given current trends toward higher functionality, higher speeds, smaller sizes and higher integration, the transistors, diodes and other semiconductor devices that are used in electronic equipment such as converters and power supplies have come to generate large amounts of heat themselves. The temperature of the equipment rises owing to such heat, triggering malfunctions and breakdowns. To address this problem, many heat-dissipating methods for suppressing temperature rises by semiconductor devices during operation, and heat-dissipating members to be used for this purpose, have been described.
For example, heat sinks that employ aluminum, copper or other metal plates having a high thermal conductivity are used to hold down the temperature rise by semiconductor devices during operation in electronic equipment. Such a heat sink carries away the heat generated by the semiconductor device and discharges the heat from a surface by utilizing the temperature difference with outside air. At the same time, the semiconductor device and the heat sink must be electrically insulated from each other. A plastic film or the like is placed between the heat-generating electronic component and the heat sink for this purpose. However, because plastic film has an exceedingly low thermal conductivity, it greatly hinders heat transfer to the heat sink. Also, in cases where a semiconductor device such as a transistor is fastened with screws to the heat sink, the screws must pass through the plastic film. Holes are formed in the film for this purpose, but such holes can lead to tearing of the film, making it impossible to maintain the electrically insulating properties. The inability to maintain the insulating properties is lethal to transistors and diodes.
To help prevent tearing and impart heat conductivity, heat-conductive sheets composed of a glass cloth with a heat-conductive resin laminated thereon have been developed. One example is a heat-conductive sheet made up of a glass cloth having laminated thereon a silicone rubber that contains boron nitride powder and spherical silica powder as heat-conductive fillers (Patent Document 1: JP-A H9-199880). During production, this sheet is subjected to applied pressure. In order to apply pressure, it is necessary to prepare a sheet of a suitable size and to use a pressing machine or the like. Because this is a batch production process, the completed heat-conductive sheet cannot be taken up into a roll, which is very inefficient from the standpoint of productivity and yield. Moreover, there are limitations on the size of the original sheet, which also imposes constraints on the size of the heat-conductive sheet during device packaging.
Continuous forming processes are exemplified by coating methods. For example, a heat-conductive sheet can be continuously produced by formation that involves sealing one side of a glass cloth with a heat-conductive silicone resin and then coating the sealed glass cloth with a layer of heat-conductive silicone rubber. In such a coating operation, the completed sheet can be continuously taken up, making production highly efficient. Also, even though the sheet width in the transverse direction is limited by the size of the coating machine, because there are no limitations on the sheet length in the machine direction, the degree of freedom in the size of the sheet during device packaging increases greatly compared with pressing. However, compared with pressing, a coating operation gives a poor surface precision, resulting in increased contact thermal resistance. Also, because pressure is not applied, the density of the silicone rubber layer tends not to increase. Hence, coating is unsuitable as a method for producing sheet having a high heat conductivity.
One way to achieve a good surface precision even with a coating process is the method of using a spherical heat-conductive filler to improve the flowability of the heat-conductive silicone resin. However, the grades of spherical heat-conductive fillers are limited, and there are various problems with each grade. For example, spherical alumina is relatively inexpensive, but because it has a high hardness, abrasion of the mixing tank arises during compounding, making maintenance of the production facilities more difficult and lowering the electrical insulating properties of the product. Agglomerated spherical powders such as aluminum nitride and boron nitride also exist, but these are very expensive, increasing the production costs of the heat-conductive sheet.
As for non-spherical heat-conductive fillers such as aluminum hydroxide which have relatively low material costs and possess electrical insulating properties, a drawback is that, because they have a large specific surface area and the amount in which such fillers can be loaded into silicone resin is limited, the thermal conductivity is low. Also, given that such fillers have a poor flowability, good surface precision is not obtained by coating and the contact thermal resistance worsens.